The modern history of Knicks basketball is defined by countless sad facts, but this particular one is a doozy: They have not drafted, developed and kept a true star player since they took Patrick Ewing first overall in 1985.

Ponder that for a second. Ronald Reagan was in the White House the last time the Knicks picked a player deemed worthy of being a franchise centerpiece over the long haul. Mark Jackson (1987) was named Rookie of the Year and made the All-Star team in his second season, yet was traded after his fifth year to the Clippers. Kristaps Porzingis (2015) made the All-Star team in his third season, yet was traded (following a serious knee injury and a serious fracture in the player-front office relationship) before he could compete in a fourth.

That’s about as good as it gets, at least until you arrive at draft day, 2019, when the Knicks selected Duke’s RJ Barrett third overall. He was supposed to be a consolation prize for Knicks fans posterized by the lottery gods who denied them the first pick and Barrett’s transformational college teammate, Zion Williamson. It hasn’t worked out that way, given the state of Williamson’s health and happiness — or substantial lack thereof — in New Orleans.

If Barrett might never be the kind of explosive game-changer the No. 2 overall pick, Ja Morant, is for Memphis, he does have a chance to be a big-time NBA player. Yes, any 21 year old who can come off an injury and immediately drop 46 points on the Miami Heat has a chance to be pretty special. Barrett should remember that over the final 20 games in yet another Knicks season from hell.

Wednesday night, he was good for 30 points and seven assists in a 123-108 loss in Philadelphia, the Knicks’ 16th defeat in their last 19 games. The visitors were up 16 points in the first half and then got blasted by a much better team in the third quarter.

“In the beginning of the third, I was terrible,” said Barrett, who conceded that a couple of his mistakes swung the momentum the Sixers’ way. The Knicks’ margin for error, their coach said, has grown painfully small.

“We’re doing some good things,” Tom Thibodeau said, “but we’re not doing enough of them to win.” On the bright side, at least Joel Embiid didn’t drop 100 points on the Knicks the way Philly’s Wilt Chamberlain scored 100 on them 60 years ago to the night.

The Knicks are 25-37 and staring at another six consecutive road games with the Suns, Clippers, Kings, Mavericks, Grizzlies and Nets that will surely end any hope of stealing a play-in bid — not that a play-in bid is an acceptable goal for a team that went 41-31 last year, claimed the Eastern Conference’s fourth seed and won over the hearts of millions of New Yorkers.

Millions of New Yorkers who feel duped right about now.

Barrett outscored the offensive juggernaut he was assigned to cover, James Harden, who was making his home debut in Philly, and of course it didn’t matter. Nearly everything the Knicks have touched this season has turned to you-know-what. Their roster was so poorly constructed that one injury to a 33-year-old reserve guard, Derrick Rose, severely compromised the plan of taking another step closer to legitimate contention. Julius Randle and Thibodeau haven’t been what they were last year, and so the Knicks will start Year 3 of the Thibs program (assuming he survives, which he should) almost right back where they were at the start of Year 1.

With one exception: Barrett appears to be an emerging long-term force. “I don’t think anybody is surprised or should be surprised,” Miami’s Jimmy Butler said last week after Barrett’s career-high 46-point outburst. “He’s definitely going to be … the face of the Knicks.”

The face of a franchise that has been faceless for far too long.

It’s clear that Barrett, not the 27-year-old Randle, will be the keeper of the current group, the one most likely to outlast all the changes the Knicks need to become legit. Somehow and some way, Barrett has risen above the unholy mess around him. He can’t stop doing that now, with five weeks left in the season. He needs to keep getting better. He needs to keep proving that he will be a real star in the league, the kind who will inspire other stars to want to play with him in the coming years.

Harden, for one, has now played for two Atlantic Division teams without giving Madison Square Garden a passing thought. The Knicks never land top-tier talent because they never develop anyone to attract those players. Barrett must be the one to change that.

“I’ve always wanted to lead by example,” he said the other day, explaining how he’s “always trying to lift [teammates’] spirits up.” Barrett needs to keep his own spirits up, too.

He can’t let his hopeless team prevent him from giving Knicks fans hope.